Locating an anatomical cavity in a body, for example the body of a patient, is important, inter alia, for anaesthetics, in which it is often necessary for substances which have an anaesthetic action to be introduced into the anatomical cavity, such as for example the epidural cavity which is located in the vicinity of the spinal cord. To locate the anatomical cavity, it is generally known to use a medical localisation device such as a hollow needle and a reservoir filled with an isotonic liquid or with a gas mixture, and often an injection syringe with a displaceable plunger. When the point of the hollow needle reaches the anatomical cavity, the fluid flowing out of the needle is no longer subject to any resistance from surrounding tissue, and the pressure in the liquid drops. The person who is handling the assembly can feel this in the hand which he is using to operate the injection syringe. These pressure variations during handling of the assembly can also be fed back to the user by means of pressure information presented on a screen or by sound. For this purpose the assembly is provided with sensor means that measure the pressure variation of the liquid and generate a pressure-signal.
An example of an apparatus for locating anatomical cavities is disclosed inter alia in EP 0 538 259.
The known apparatus comprises a hollow needle, a fluid-filled reservoir which is in communication with the needle, pump means for pressurizing the fluid, measuring means for creating a pressure-measurement signal which is related to the pressure prevailing in the fluid, signal-conversion means for converting the pressure-measurement signal created by the measuring means into a form which is suitable for further processing, and reproduction means for emitting an acoustic signal which is related to the pressure-measurement signal which has been converted by the signal-conversion means.
The needle of the known apparatus is intended to be introduced into a body and is connected to the reservoir in the form of an injection syringe. The injection syringe contains fluid in the form of an isotonic liquid. The needle and the injection syringe are in communication with one another via a T-shaped connector. The pressure-measuring means, which are used to detect and measure the pressure prevailing in the liquid in the injection syringe, are also connected to this T-shaped connector and is in communication with the needle and injection syringe. The known apparatus also comprises a processor for processing a pressure-measurement signal created by the pressure-measuring means, in order that the rate of pressure variation can always be determined when in communication with the needle and the injection syringe, which pressure variation is primarily the consequence of the different tissue encountered when moving the needle in the body.
When the known apparatus is being used, the starting point is a situation in which the point of the needle is already situated in the vicinity of the cavity which is to be located. The needle, the injection syringe and the pressure-measuring means are set in communication to one another by changing the position of a switch in the T-shaped connector. In the injection syringe there is a plunger which functions as a pump means for displacing the liquid through the needle and thus produces pressure in the liquid. The pressure which is shown on the screen when there is no pressure being exerted on the plunger of the injection syringe is calibrated to zero. Then, the person who is handling the injection syringe and the needle brings the pressure in the liquid in the injection syringe to a defined level by exerting pressure on the plunger. During this process, he can read the level of the pressure from the screen at any time. When the liquid in the injection syringe has been brought to the required pressure, the person who is handling the device can move the needle towards the cavity in the body while using the pressure data displayed on the screen to carefully maintain a pressure on the plunger. When the level of the pressure variation exceeds a minimum level stored in the processor and/or the pressure variation rate over a defined time period is within minimum margins stored in the processor, the warning means are activated and emit a first, acoustic warning signal via acoustic reproduction means. If the pressure can be restored by slightly displacing the plunger, without further displacement of the needle, the first acoustic warning signal will stop. On the other hand, if a more abrupt pressure variation occurs and the pressure cannot be restored by displacing the plunger, the warning means emit a second acoustic warning signal, which clearly differs from the first warning signal. From the second warning signal, the person who is handling the device can infer that the point of the needle has reached the anatomical cavity and the he must stop moving the needle.
A drawback of the apparatus and method which are known from EP 0 538 259 is that it is unknown to the person who is handling the injection syringe if the measuring means and/or the signal-conversion means are working correctly. If the measuring means and/or the signal conversion means are defect, for example they give erroneous output or do not work at all, the instantaneous pressure data on the screen is shown wrongly. Moreover, emitted acoustic warning signals do not match with the actual pressure. It may also be possible that no warning signal is emitted at all even when the level of the pressure variation exceeds the minimum level stored in the processor and/or the pressure variation rate over a defined time period is within the minimum margins stored in the processor. This results in a handling of the syringe that can be dangerous. For example, the person who handles the syringe will pass the needle beyond the cavity and affect the spinal cavity or spinal cord which have particularly adverse results for the patient.